The present invention relates to systems for the production of olefins by pyrolysis of hydrocarbon feedstocks and more particularly a low pressure chilling process and systems for separating hydrogen and methane.
The production of olefins involves the thermal cracking of a variety of hydrocarbon feedstocks ranging from ethane to heavy vacuum gas oils. In the thermal cracking of these feedstocks, a wide variety of products are produced ranging from hydrogen and methane to pyrolysis fuel oil. The effluent from the cracking step, commonly called charge gas or cracked gas, is made up of this full range of materials which must then be separated by fractionation into various product and by-product streams followed by hydrogenation of at least some of the unsaturated by-products.
In the majority of operating units, the cracked gas is compressed from approximately 1 to 1.4 bars up to 27 to 42 bars. The purpose of this compression is to permit the separation of hydrogen and methane from the C.sub.2 and heavier components contained in the cracked gas. Generally, the cryogenic portion of the plant consists of chilling the relatively high pressure compressed gas by mechanical refrigeration and other cold process streams thereby condensing all the C.sub.2 and heavier components. In addition, the compression permits the delivery of high purity hydrogen to the downstream hydrogenation processes at high pressures. This compression and cryogenic separation of the materials in the cracked gas is a very energy intensive and high capital investment process.